The invention relates to positive-working lithographic printing plates. More particularly, it relates to methods for avoiding the need to remove unwanted, unexposed areas left on the finished plates due to shading of sections of the plate precursors by platesetter clamps or other plate-holding elements.
In lithographic printing, ink-receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink-receptive regions accept the ink and repel the water. The ink is then transferred to the surface of a material upon which the image is to be reproduced. Typically, in a method known as xe2x80x9coffsetxe2x80x9d, this is done indirectly by first transferring the ink to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A class of imageable elements called printing plate precursors, useful for preparing lithographic printing plates, comprises a layer applied over the surface of a hydrophilic substrate. The layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, or in addition, the binder itself may be radiation-sensitive. The layer is commonly applied as a coating, using a solvent.
If after exposure to radiation the exposed regions of the coating are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate precursor is referred to as xe2x80x9cpositive-workingxe2x80x9d. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate precursor is called xe2x80x9cnegative-workingxe2x80x9d.
In both cases, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
An alternative way of achieving the same result is to begin with a hydrophilic surface upon which, after imagewise exposure and developing, an ink-receptive pattern representing the image is obtained. If the unexposed areas become ink receptive, the plate precursor is xe2x80x9cpositive-workingxe2x80x9d, while if the exposed areas become ink receptive, it is xe2x80x9cnegative-workingxe2x80x9d.
Recent developments in the field of printing plate precursors deal with radiation-sensitive compositions that can be imagewise exposed by means of lasers or laser diodes. This type of exposure, known as digital imaging, does not require films as intermediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes that are typically used in commercially available exposure devices (known as platesetters) emit light in the wavelength ranges of either 800 to 850 nm or 1060 to 1120 nm. Therefore printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such platesetters, have to be sensitive in the near infrared range. They are not however typically very sensitive to visible light. Such printing plate precursors can therefore basically be handled under daylight conditions, which significantly facilitates their production and processing.
Thermally imageable elements useful as lithographic printing plate precursors, exposable by infrared lasers or laser diodes as described above, are becoming increasingly important in the printing industry. After imagewise thermal exposure, the rate of removal of the exposed regions by a developer in positive-working elements is greater than the rate of removal of the unexposed regions, so that the exposed regions are removed by the developer to form an image. Such systems are disclosed in, for example, Parsons, WO 97/39894 and U.S. Pat. No. 6,280,899; Nagasaka, EP 0 823 327; Miyake, EP 0 909 627; West, WO 98/42507 and U.S. Pat. No. 6,090,532; and Nguyen, WO 99111458 and U.S. Pat. No. 6,060,217.
Printing plate precursors are also in use which are imageable by ultraviolet radiation, as are types that are imageable by visible radiation.
Imaging of digital, thermally imageable precursors is typically done using platesetters, where the plate precursor is mounted either
i). on a rotatable drum (external drum), typically using clamps, or
ii). in a drum (internal device), in which case the plate precursors are held in place with compressed air or with clamps, which may be magnetic.
When a positive-working lithographic printing plate precursor is imaged on a platesetter employing clamps, the clamping device prevents the successful exposure of the coating immediately under the clamp. After development, this unexposed area of coating accepts ink. Unless this section of coating is removed manually (a time-consuming process), it will cause an unwanted image on the press. The problem is particularly troublesome for web presses, where ink is wasted and unwanted inked image areas can transfer to the back of paper stocks.
Rather than using clamps, some platesetters employ suction cups and powerful vacuums. On mounting a plate precursor on such a platesetter, however, at least one edge of the plate precursor is typically inserted into a crevice in the drum, where it is shaded from the imaging radiation. In such systems, the presence of unwanted, remaining image areas is therefore still not avoided. Thus there remains a need for ways of avoiding the time-consuming step of removing such unwanted image areas after plate development.
This need is addressed by the present invention. In one aspect, the invention is a method for eliminating an unwanted ink-receptive section of a printing plate precursor present after treatment of an unwanted unexposed section with a developer, the unwanted ink-receptive section being shaded by a plate-holding element from exposing radiation during an imagewise exposure of the plate precursor and therefore remaining unexposed to the exposing radiation following the imagewise exposure, the section comprising a heat-sensitive layer, the method comprising heating the unwanted unexposed section prior to developing the plate precursor to a temperature such that following development the section is hydrophilic.
In another aspect, the invention is a method for preparing a printing plate comprising the steps of
(a) providing a positive-working printing plate precursor comprising a radiation-sensitive region on a front surface of the precursor, the radiation-sensitive region comprising a heat-sensitive layer;
(b) mounting the precursor on an exposure device with a plate-holding element wherein the element overlaps a section of the radiation-sensitive region of the precursor;
(c) imagewise exposing the radiation-sensitive region of the precursor to exposing radiation, the radiation selected to render the heat-sensitive layer exposed to the radiation soluble or dispersible in a developer;
(d) heating the section of the radiation-sensitive region overlapped by the element in a manner sufficient to render the overlapped section soluble or dispersible in the developer;
(e) removing the precursor from the exposure device; and
(f) developing the precursor in the developer to form the plate.
In yet another aspect, the invention is a method of avoiding the formation of an unwanted unexposed section in a positive-working printing plate precursor due to shading by a plate-holding element from exposing radiation during an imagewise exposure of the precursor and therefore remaining unexposed to the exposing radiation following the imagewise exposure, the method comprising employing a plate-holding element to secure the precursor during imagewise exposure, wherein the plate-holding element is substantially transparent to the exposing radiation.
In still another aspect, the invention is a method for preventing the generation of ink receptivity in an unwanted unexposed section of a developed plate prepared from a positive-working printing plate precursor, the section being shaded by a plate-holding element from exposing radiation during an imagewise exposure of the precursor and therefore remaining unexposed to the exposing radiation following the imagewise exposure, the section comprising a heat-sensitive layer capable of generating ink receptivity in the developed plate upon treatment of the precursor with a developer, the method comprising heating the section prior to the treatment, the heating selected to render the heat-sensitive layer of the section incapable of generating ink receptivity in the developed plate upon treatment of the precursor with the developer.
In a still further aspect, the invention is a method for preparing a printing plate comprising the steps of
(a) providing a positive-working printing plate precursor comprising a radiation-sensitive region on a front surface of the precursor, the radiation-sensitive region comprising a heat-sensitive layer capable, upon treatment with a developer, of generating ink receptivity in a developed plate prepared from the precursor;
(b) mounting the precursor on an exposure device with a plate-holding element wherein the element overlaps a section of the radiation-sensitive region of the precursor;
(c) imagewise exposing the radiation-sensitive region of the precursor to exposing radiation, the radiation selected to render the heat-sensitive layer exposed to the radiation incapable of generating ink receptivity in the developed plate upon treatment of the precursor with the developer;
(d) heating the section of the radiation-sensitive region overlapped by the element, the heating selected to render the heat-sensitive layer of the section incapable of generating ink receptivity in the developed plate upon treatment of the precursor with the developer;
(e) removing the precursor from the exposure device; and
(f) developing the precursor in the developer to form the plate.
One process of producing a printing plate from a positive-working printing plate precursor involves providing a precursor, imagewise exposing it to radiation designed to make exposed parts of the radiation-sensitive layer soluble or dispersible in a developer, and using the developer to produce a finished plate. In the present invention, unwanted unexposed areas can also be rendered soluble or dispersible through selective heating, or through avoiding their formation altogether via the use of plate-holding elements that are substantially transparent to the exposing radiation. Each of these elements will be discussed in detail below.
A variety of printing plate precursors is available commercially. Depending on the type of precursor, the imaging radiation is commonly visible radiation, ultraviolet radiation, or infrared radiation, with precursors of this last type also being called xe2x80x9cthermalxe2x80x9d plate precursors.
Thermal plate precursors are characterized by the presence of a xe2x80x9cphotothermal conversion materialxe2x80x9d which absorbs the imaging radiation and converts it to heat, causing imaged areas of the precursor to become soluble or dispersible in the developer. Photothermal conversion materials may absorb ultraviolet, visible, and/or infrared radiation to perform this function. Such materials are disclosed in numerous patents and patent applications, including Nagasaka, EP 0,823,327; Van Damme, EP 0,908,397; DeBoer, U.S. Pat. No. 4,973,572; Jandrue, U.S. Pat. No. 5,244,771; and Chapman, U.S. Pat. No. 5,401,618. Examples of useful absorbing dyes include ADS-830 WS and ADS-1064 (both available from American Dye Source, Montreal, Canada), EC2117 (available from FEW, Wolfen, Germany), CYASORB(copyright) IR 99 and CYASORB(copyright) IR 165 (both available from Glendale Protective Technology), EPOLITE(copyright) IV-62B and EPOLITE(copyright) III-178 (both available from the Epoline), PINA-780 (available from the Allied Signal Corporation), SpectralR 830A and SpectralR 840A (both available from Spectra Colors).
Plate precursors useful for this invention include 1-layer thermal plate precursors, which are a preferred embodiment. These are commercially available under such trade names as ELECTRA(copyright) and ELECTRA(copyright) EXCEL, available from Kodak Polychrome Graphics. Single layer thermal plate precursors are described by Parsons, U.S. Pat. No. 6,280,899, incorporated herein by reference.
Also preferred are 2-layer systems in which the photothermal conversion material resides in the bottom layer. Such a system is commercially available under the trade name SWORD(trademark), available from Kodak Polychrome Graphics. Systems of this sort are described by Shimazu in U.S. Pat. No. 6,352,812 and by Savariar-Hauck in U.S. Pat. No. 6,358,669, both incorporated herein by reference, and comprise a hydrophilic substrate, an underlayer on the substrate which comprises a developer-soluble or developer-dispersible polymer and a photothermal conversion material, and a top layer that is not soluble or dispersible in the developer.
Also useful for this invention are 2-layer thermal plate precursors in which the photothermal conversion material resides in the top layer. These are described for instance by Van Damme, EP-O-864-420-A1 and Verschueren, EP-O-940-266-A1.
Three-layer thermal plate precursors are also useful, such as are described in U.S. application Ser. No. 09/999,587, incorporated herein by reference. Such systems comprise a hydrophilic substrate, an underlayer on the substrate which comprises a developer-soluble or developer-dispersible polymer and a photothermal conversion material, a barrier layer to prevent the photothermal conversion material from migrating, comprising a developer-soluble or developer-dispersible polymer, and a top layer comprising a polymer that is not soluble or dispersible in the developer.
Also useful for this invention are 2-layer visible light sensitive plate precursors, of which a number of models are well known and commercially available.
Another type of printing plate precursor suitable for use with this invention is described by Watkiss in U.S. Pat. No. 4,859,290. In such a system, unexposed silver halide diffuses to the surface of an aluminum substrate bearing nuclei capable of reducing the silver halide to metallic silver, which forms the basis for an oleophilic region on the developed plate. In this system, the silver halide in exposed areas is incapable of such diffusion and thus does not render the substrate oleophilic. According to the present invention, such immobilization of the silver can also be achieved by heating unexposed sections of the precursor.
Although the above-mentioned systems are the most common, the invention is applicable to radiation-sensitive positive-working systems irrespective of the number of layers employed in the plate precursor, and irrespective of whether the hydrophilic areas of the finished plate are formed by removal of hydrophobic material or by preventing the conversion of hydrophilic areas to ink-receptive ones. In general, these precursors are all employed in their routine manner of use, except where explicitly deviated from for the purposes of the invention.
Imaging of the precursors can be performed with commercially available exposure devices, also known as platesetters. For thermal systems, for example, one can use a Creo TRENDSETTER(copyright) 3244, supplied by CreoScitex Corporation, Burnaby, Canada; a Platerite 8000, supplied by Screen, Rolling Meadows, Ill.; or a Gerber Crescent 42T, supplied by the Gerber Corporation. Many others are available, and any of these is applicable. The platesetter is used according to normal procedures for the unit, except where explicitly deviated from for the purposes of the invention. Typical exposure conditions for thermal plate precursors are given in the Examples.
For platesetters using visible light, commercial units include Platerite from Screen, Rolling Meadows, Ill.; LaserStar from Krause, Branford, Conn.; Antares 1600 from Cymbolic Sciences, Blaine, Wash.; Galileo from Agfa, Wilmington, Mass.; and Lithosetter III from Barco Graphics, Vandalia, Ohio.
The transparent plate-holding elements are constructed of materials that are substantially transparent to the imaging radiation; such materials are well known in the art. Suitable materials of construction of the plate-holding elements include, but are not limited to, most grades of clear glass, polymethyl methacrylate, polycarbonate, polyvinyl chloride, glass fiber-reinforced polyester, magnesium fluoride, barium fluoride, calcium fluoride, potassium bromide, and lithium fluoride. Also useful are thallium halides, especially mixtures such as 1) about 40 wt. % thallium bromide and about 60 wt. % thallium iodide, and 2) about 30 wt. % thallium bromide and about 70 wt. % thallium chloride. Also useful are chalcogenide glasses, polycrystalline zinc selenide, zinc sulfide, and lanthanide sulfides. Fused silica (isotropic silicon dioxide) is also useful.
The printing plate precursors of this invention are positive working, meaning that the radiation-sensitive composition is ultimately removed from areas exposed to the imaging radiation. This requires that the composition in those areas be converted to a form that is more easily soluble or dispersible in the developer than it is in the unexposed areas. In the case of infrared-sensitive plate precursors, heating of the exposed areas causes this change, and is usually performed by the action of an infrared laser during the imaging process. If the plate-holding elements are largely opaque to the infrared radiation, areas of the precursor under them do not get heated and therefore cannot normally be removed during developing. However, according to this invention these areas are heated by other means, thereby rendering these areas also removable by the developer.
The time and temperature of heating required for the practice of this invention vary with the sensitivity of the plate precursor to thermal energy, with the time required being a roughly inverse function of the temperature applied. Such conditions are easily determined for a given type of plate precursor.
Several different methods can be used to perform the heating step. Examples include, but are not limited to, the following methods.
Heating may be performed by contact of the precursor with a hot bar of such a size and shape as to contact the shaded areas of the precursor, without contacting any areas within the imaged region. The bar may be incorporated in a frame or other holding device separate from the platesetter, or may be incorporated into the platesetter in the form of the plate-holding element itself.
The bar may be made of any of a number of materials, providing that the material be thermally stable under the conditions of use and resistant to attack by any chemical components, e.g. from the plate precursor, with which it might come into contact.
Preferred are materials that have a high heat capacity and are highly thermally conductive, for example stainless steel and copper.
Advantageously, the bar may be heated internally by electrical resistance.
Heating may also be performed by the use of a strip infrared heater, such as Model FB or FBG from Casso-Solar Corporation, Pomona, N.Y.; Series TRH or series CV infrared heaters from Infrared Heaters, Clearwater, Fla.; and ProDryer or Ram-Dryer infrared ovens from IR Systems, Jupiter, Fla.
Heating may also be performed by using a stream of hot air, directed in such a way as to heat the shaded regions of the precursor but not the image area.
Heating can be performed before the plate precursor is put on the platesetter, if it is known in advance what areas will be shaded from the imaging radiation by the plate-holding elements. Advantageously, such pre-treatment may be performed at the time of precursor manufacture, eliminating the need to take additional steps at the time of use.
Heating can also be performed on the platesetter itself, using plate-holding elements that are heated by some means, an example being electrical resistance heating. The use of this approach allows the treatment to be performed while the precursor is on the platesetter for imaging, so that the heat treatment and the imagewise exposure are substantially simultaneous.
Alternatively, heating can be performed after imagewise exposure, on a device constructed for the purpose. In this mode of practice, existing commercial platesetters can be used without modification, while still allowing expeditious removal of the unwanted unexposed areas.
It is also possible to perform heating even after the plate precursor has been fully developed, which requires however that the developing step be subsequently repeated.
Developing of the exposed precursors to form the finished plates is performed with commercially available developers designed for the type of plate precursor being used. Many types are available, and their selection and use is well known in the art. Essentially any developer normally suitable for use with a particular plate precursor is suitable for use in the practice of this invention. In general, normal procedures are used unless specific mention is made to the contrary.